I have to agree with OFL...it's the size of the tank and whether it's planted that decides if there will be problems. I have kept 3 female bettas in a 10g with guppies and they each interact with their species and pretty much ignore the other even during feeding.
I have also kept a male betta in a planted 29g with guppies, neons, cories, and even rainbow fish...I have never had cross species issues...sometimes it will get a bit boisterous with the schools of fish playing tag (especially with mollies!) But when that happens, Sapphire just moves to a quiet corner and sits in a plant...sometimes I swear he is watching his version of sitcom tv and laughing his little monkey face off!
It seems to hinge on whether tank is large enough for each species to have a place to call their own and retreat to. I think it is also important to remember line of sight...if a weaker, picked on species can disappear from constant view of an aggressive fish, it should be ok...kind of like when you are attempting a spawning...if one is not ready and the male gets violent, the female has to have a hiding place or cave to retreat to for a rest...the male will usually leave her alone if she is far enough away from nest and not in his direct view.
Don't get me wrong...you still have to monitor any fish in a new situation and act accordingly to protect fish that can't get away!